THE XKTLL. 



481 



V.I 



c fr 



Frog stands, so far as development, is concerned, in striking contrast 

 to the mandibular arch of the Axolotl, in spite of the obvious 

 similarity in the arrangement of the adult parts in the two types. 



In the earliest stage it forms a simple bar in the membranous 

 mandibular arch, parallel to and very similar to the hyoid bar behind 

 (fig. 337, Mn). In the next stage observed, that is to say in Tadpoles 

 of four, five, to six lines long, an astonishing transformation has 

 taken place. The mandibular arch (fig. 338) is turned directly 

 forwards parallel to the trabecula, 

 to which it is attached in front 

 (p.pf/) and behind (pd). The 

 proximal part of the arch thus 

 forms a subocular bar, and the 

 space between it and the trabe- 

 cula a subocular fenestra. In 

 front of the anterior attachment 

 it is continued forwards for a 

 short distance, and to the free 

 end of this projecting part is 

 articulated a small Meckelian 

 cartilage directed upwards (ink). 

 The Meckelian cartilage is at 

 this stage placed in front of the 

 nasal sacks, in the lower lip of 

 the suctorial mouth. The greater 

 part of the arch, parallel with 

 the trabecul*, is equivalent to 

 what has been called in the Axo- 

 lotl the quadrate, while its an- 

 terior attachment to the trabe- 

 cuht! is the rudiment of the 

 palato-pterygoid cartilage. The 

 posterior attachment is known as trabecute; p.pf,. palatopterygoid bar; P ,i. 



r . ,. , pedicle; q. quadrate condyle ; ink. Meck- 



tne pedicle. elian piece of mandibular arch; s.o.f. 



m , ,.-. , Ti i subocular fenestra; -it. I. upper labial car- 



Ihe condition of the mandibular tila ^ The dotted circle within the qua . 



arch during this and the next stage drate region indicates the position of the 



(fig. 339) is very perplexing. Its internal nostril. 



structure appears adapted in some 



way to support the suctorial mouth of the Tadpole. 



Reasons have been offered in a previous part of this volume for sup- 

 posing that the suctorial mouth of the Tadpole is probably not simply a 

 structure secondarily acquired by this larva, but is an organ inherited from 

 an ancestor provided through life with a suctorial mouth. 



The question thus arises, is the peculiar modification, of the mandibular 

 arch of the Tadpole an inherited or an acquired feature ? 



If the first alternative is accepted we should have to admit that the 

 mandibular arch became first of all modified in connection with the 

 suctorial mouth, before it was converted into thr> jaws of the Guatho- 



ni.s '- 



FIG. 33S. TADPOLE OF COMMON TOAD, 



ONE-THIHD OF AN INCH LONG ; CRANIAL AND 

 MANDIBULAR CARTILAGES SEEN FEOM ABOVE ; 

 TUE PARACHORDAL CARTILAGES ARE NOT YET 



DEFINITE. (From Parker.) 



nc. notochord ; ms. muscular segments ; 

 an. auditory capsule; py. region of pi- 

 tuitary body; tr. trabecula; <.//. cornu 



B. E. II. 



